1. Field of the Invention
The present invention relates to an image forming apparatus using multiple beams, and more particularly, to stabilization of the density of a formed image.
2. Description of the Related Art
In a conventional image forming apparatus using an electrophotographic method in which a visual image is formed according to charging, exposure and development, an approach has been adopted widely in which, after performing primary charging of an electrophotographic photosensitive member, which serves as an image bearing member, exposure is performed, using a semiconductor laser as means for forming an electrostatic latent image on the photosensitive member. (To be more exact, a laser chip comprising a laser diode and a photodiode sensor is used as the means for forming an electrostatic latent image.) By feeding back an output signal from the photodiode sensor to a bias power supply for the laser diode, and automatically controlling the amount of the bias current, a laser beam is stabilized.
Recently, in order to realize high-speed printing with an image forming apparatus, means for forming an electrostatic latent image using a multi-laser device has been practically used, in which a plurality of laser beams are simultaneously emitted at one main scanning operation. For example, in a multilaser method using two lasers, the above-described configuration is adopted: that is, each laser comprises a pair of a laser diode and a photodiode sensor, in order to stabilize the obtained laser beam.
Various image-signal processing techniques for improving the quality of an image are being used. For example, a method has been proposed in which, when forming an image by binary coding a digital image signal, the digital image signal is first converted into an analog signal, and a binary signal subjected to pulse-width modulation (PWM) is generated by comparing the analog signal with a periodic pattern signal, such as a triangular-wave signal. An invention in which the above-described PWM method is applied to a multibeam laser printer is disclosed in Japanese Patent Application Laid-Open (Kokai) No. 8-317157 (1996). In this invention, in order to prevent variations in the image density due to individual differences among multiple laser beams, a pattern signal for each laser is corrected by PWM. That is, each laser beam is sometimes subjected to peculiar PWM in accordance with the characteristics of the beam, so that variations in the image density are suppressed by providing a uniform light-portion potential by laser scanning, by reducing variations in the amounts of light output by the respective laser beams.
However, in a multibeam laser printer, there is the problem that the halftone image density differs even if variations in the characteristics of the laser beams are not present. This is a new problem such that the halftone image density differs if at which the position to start image writing shifts even just by one line in the sub-scanning direction. It is considered that this phenomenon is caused by nonlinearity of the curve of the amount of light E of the photosensitive member versus the potential V (Exe2x88x92V curve). For example, the amount of light E is expressed by E=Ixc3x97t, where I is the intensity of light, and t is the exposure time. The above-described variations in the density are produced because, even if the same amount of light E is provided for the photosensitive member, the sensitivity differs and the potential may change if the intensity of light I changes or the exposure time t changes. This phenomenon is called reciprocity. With respect to reciprocity, an example in which the sensitivity increases by projecting weak light beams onto a photosensitive member a plurality of times is reported in Japanese Patent Application Laid-Open (Kokai) No. 4-51043 (1992).
An example of differences in the halftone density caused by reciprocity in multiple beams will now be illustrated. FIG. 15 is a schematic diagram illustrating a halftone image with two dots and two spaces obtained by simultaneously projecting beams A and B. A pair of laser beams are defined as beams A and B. The beam A corresponds to the first line of writing positions on paper, and the beam B corresponds to the second line. Thereafter, the beams A and B are alternately projected on odd lines and even lines, respectively. After the beams A and B simultaneously are in an on-state to scan two-dot lines at the first scanning by a polygonal mirror, the beams A and B simultaneously assume an off-state turned of at the next scanning by the polygonal mirror, to provide two spaces. A halftone image with two dots and two spaces is obtained by sequentially repeating simultaneous on-state and off-state of the beams A and B. In FIG. 15, the pairing of the laser beams is indicated by being surrounded by broken lines.
FIG. 16 is a schematic diagram illustrating a halftone image with two dots and two spaces obtained by simultaneously (sequentially) projecting the beams A and B. At the first scanning by the polygonal mirror, the beam A is in an off-state and the beam B is in an on-state, to provide a one-dot line and one space. At the next scanning by the polygonal mirror, the beam A is in an on-state and the beam B is in an off-state, to provide a one-dot line and a one-dot space. A halftone image with two dots and two spaces shifted by one line is obtained by sequentially repeating the above-described one space and one dot, and one dot and one space.
The densities of the images with two dots and two spaces shown in FIGS. 15 and 16 were compared with each other. The density of the image with two dots and two spaces shown in FIG. 15 in which the two laser beams were simultaneously projected in the main scanning direction, was 1.15. The density of the image with two dots and two spaces shown in FIG. 16 in which the laser beams were alternately projected, was 1.21. Accordingly, the density in simultaneous irradiation is lower than the density in alternate irradiation.
In order to study the reason for this difference, first, it was checked if a difference was present in the amount of light. It can be considered that the amount of light may decrease during simultaneous irradiation due to mutual influence between the laser beams caused by thermal and electrical crosstalk between the laser beams. Accordingly, the amount of light of laser beams when a pair of laser beams were simultaneously projected and the amount of light of a laser beam when a single laser beam was projected were measured and compared with each other.
FIG. 17 is a graph illustrating the value of the amount of light measured by a pin-photodiode when only the beam A performed scanning. FIG. 18 is a graph illustrating the value of the amount of light measured by the pin-photodiode when only the beam B performed scanning. FIG. 19 is a graph illustrating the value of the amount of light measured by the pin-photodiode when the beams A and B were simultaneously emitted to perform scanning. The sum of the amounts of light of the beams A and B shown in FIGS. 17 and 18, respectively, coincides with the amount of light of simultaneous emission of the beams A and B shown in FIG. 19. This result indicates that the amounts of light of multiple beams are not reduced and stable even if the two beams are simultaneously emitted.
Next, it was studied if there is a difference in the potential of the photosensitive member. Since the diameter of the used laser spot is not small, it is estimated that superposition of spots of a pair of laser beams occurs, and the potential differs at a superposed portion. Scanning was performed in the conditions that the beams A and B had the same spot diameter with the size of 70 xcexcm both in the main scanning and sub-scanning directions. The size of one pixel of an image with a resolution of 1,200 dpi (dots per inch) was 21 xcexcm.
FIG. 20 is a schematic diagram illustrating a state in which a light-amount distribution at simultaneous exposure is converted into a potential-distribution via an Exe2x88x92V curve. The amount of multibeam light obtained by superposing the beams A and B projected onto the photosensitive member is converted into a potential via the Exe2x88x92V curve. A noteworthy area is the portion where the spots are superposed. After synthesizing the amounts of light, the obtained multibeam light is projected onto the photosensitive member, where holes are simultaneously generated, and the potential distribution is determined. Even if the beams A and B slightly shift in the scanning direction, it, can be deemed that the beams A and B are simultaneously projected onto the photosensitive member because the time period of light projection is very short, i.e., about 1 xcexcsec.
FIG. 21 is a schematic diagram illustrating a state in which a light-amount distribution at individual exposure is converted into a potential distribution via the Exe2x88x92V curve. In FIG. 21, arrows (1) indicate a path through which the first-time potential-distribution is determined as a result of generation of holes after projecting the first beam A onto the photosensitive member, and arrows (2) indicate a path through which the second-time potential-distribution is determined as a result of generation of holes after projecting the next beam B onto the photosensitive member.
By comparing FIGS. 20 and 21 with each other, the following conclusion is obtained. That is, although the total amount of light is the same at a portion where the spots are superposed, at simultaneous irradiation, strong light is projected onto the photosensitive member only once, to determine the potential. Even when individually projecting weak light twice, the potential can be sufficiently reduced because the Exe2x88x92V curve is downwardly-convex nonlinear, and two potential distributions are to be superposed. However, the Exe2x88x92V curve for changing the amount of light in each of FIGS. 20 and 21 is in the case of exposure on the entire surface, and therefore is not strictly applied to a halftone image with two dots and two spaces. Accordingly, an Exe2x88x92V curve obtained with two dots and two spaces was actually measured, and it was studied whether or not there is a difference between simultaneous exposure and individual exposure in a photosensitive member having a downwardly-convex nonlinear Exe2x88x92V curve.
FIG. 22 is a graph illustrating results of measuring the surface potential of a photosensitive member by changing the amount of light with two dots and two spaces at simultaneous and individual irradiation of the beams A and B in multibeam irradiation. The values with two dots and two spaces are shown in Table 1.
The graph shown in FIG. 22 indicates that the curve of the potential versus the amount of light with two dots and two spaces at simultaneous irradiation is always higher than the curve of the potential with two dots and two spaces at individual irradiation, and therefore has a lower sensitivity. More specifically, the amount of light in the image forming apparatus was set to 3.0 mJ/m2. In the case of FIG. 15 in which a pair of beams were simultaneously projected, the potential was xe2x88x92265 V, while in the case of FIG. 16, in which a pair of beams were individually projected, the potential was xe2x88x92250 V. Since reversal development is performed, the density is lower with the potential of xe2x88x92265 V than with the potential of xe2x88x92250 V. As described above, this difference between potentials causes different densities, of 1.15 and 1.21, respectively. Accordingly, in order to adjust the density at simultaneous irradiation to the density at individual irradiation, it is necessary to increase the amount of light at simultaneous irradiation to about 9/8 times, i.e., to a value of about 3.4 mJ/m2.
As described above, in the case of multibeam irradiation, the potential at simultaneous irradiation of a pair of beams is higher than the potential at individual irradiation and therefore the sensitivity is lower even if the amount of light is the same, because of reciprocity. That is, at a portion where the beams are superposed, when the beams are simultaneously emitted, a photosensitive member is irradiated at a time in a state in which the amounts of light of the beams are added. On the other hand, when the beams are individually emitted, the photosensitive member is irradiated in a state in which the amount of light of each of the beams is individually supplied. At that time, although a halftone image is written in a state in which the line to start writing is shifted only by one line between the two cases, the sensitivity is lower in the former case, thereby causing a difference in the density.
The present invention has been made in consideration of the above-described problems.
It is an object of the present invention to provide an image forming apparatus and method in which a halftone density is stabilized irrespective of a timing to start writing of scanning lines even if multibeam irradiation is performed.
According to one aspect of the present invention, an image forming apparatus for forming an image by exposing a photosensitive member by causing a plurality of laser beams to perform simultaneous scanning, includes first determination means, for determining whether or not a portion exposed by adjacent laser beams with partial superposing or overlapping of the beams, simultaneously, is present, and second determination means, for determining whether or not a portion adjacent to the exposed portion and not exposed by a laser beam is present, if a result of determination by the first determination means is affirmative. Also included are control means, for adding exposure by fine dots to the unexposed portion, if a result of determination by the second determination means is affirmative.
According to another aspect of the present invention, an image forming, apparatus for forming an image by exposing a photosensitive member by causing a plurality of laser beams to perform simultaneous scanning includes first determination means for determining whether or not a portion exposed by adjacent laser beams with partial overlapping or superposing, simultaneously, and a portion exposed by adjacent laser beams with partial superposing, sequentially, are present, and second determination means, for determining whether or not a portion adjacent to the portion exposed by adjacent laser beams with partial superposing, simultaneously, and not exposed by a laser beam is present, if a result of determination by the first determination means is affirmative. Also provides are control means for adding exposure, by fine dots to the unexposed portion, if a result of determination by the second determination means is affirmative.
According to still another aspect of the present invention, an image forming apparatus includes latent-image forming means for forming a latent image by exposing a photosensitive member by performing simultaneous scanning with two laser beams in a main scanning direction. When the photosensitive member is exposed by the two laser beams at one scanning operation in the main scanning direction with partial superposing, the latent-image forming means adds exposure by fine dots at an immediately preceding or immediately succeeding scanning operation with respect to the one scanning operation in the main scanning direction.
According to yet another aspect of the present invention, an image forming apparatus includes latent-image forming means for forming a latent image by exposing a photosensitive member by performing simultaneous scanning with two laser beams. When the two laser beams perform simultaneous emission at one scanning operation in a main scanning direction, the latent-image forming means adds exposure by fine dots at an immediately preceding or immediately succeeding scanning operation with respect to the one scanning operation in the main scanning direction. When one of the two laser beams performing emission at the one scanning operation in the main scanning direction and one of the two laser beams performing emission at an immediately succeeding scanning operation in the main scanning direction are adjacent to each other, the latent-image forming means does not add exposure of fine dots.
According to yet a further aspect of the present invention, an image forming method in an image forming apparatus for forming an image by exposing a photosensitive member by causing a plurality of laser beams to perform simultaneous scanning, includes a step A, of determining whether or not a portion exposed by adjacent laser beams with partial superposing, simultaneously, is present, and a step B of determining whether or not a portion adjacent to the exposed portion and not exposed by a laser beam is present, if a result of determination in the step A is affirmative. In a step C, there is added exposure by fine dots to the unexposed portion, if a result of determination in step B is affirmative.
According to still another aspect of the present invention, an image forming method in an image forming apparatus for forming an image by exposing a photosensitive member by causing a plurality of laser beams to perform simultaneous scanning, includes a step A, of determining whether or not a portion exposed by adjacent laser beams with partial superposing, simultaneously, and a portion exposed by adjacent laser beams with partial superposing, sequentially, are present, and a step B, of determining whether or not a portion adjacent to the portion exposed by, adjacent laser beams with partial superposing, simultaneously, and not exposed by a laser beam is present, if a result of determination in the step A is affirmative. In a step C, exposure by fine dots is added to the unexposed portion, if a result of determination in step B is affirmative.